they come from a moth that lays eggs on the stem or leaves the hatch and the borer moves to stem eats it way in and stays until its full grown and then cocoons and goes into the ground for next year. To control I watch for a hole in the stem could be any where but mostly near base I slit the stem a little and pull them out and them hill; soil around wound and water for a few days you can also inject a Bt solution with a hypo needle and do the same with the soil.
If it's Zucchini I would replant now you should be able to get a crop before frost but you will need to watch for Squash bugs they will lay egg clusters on either side of the leaves (bronze in color) I use the sticky side of masking tape to pull them off of the leaves for the adults I use a squirt bottle with a soapy watery mix ( I use Dawn dish soap a few drops to0 a Qt. of water ) you must hit them with the spray and it won't affect the eggs the adults and nymphs will die in 2-3 minute. good luck

I have the squash bugs not the borers and they have killed two squash plants and left one that looks pitiful. Now they have moved to the cucumbers. I will try the masking tape and Dawn, thanks. My tomatoes are the most pitiful I have ever grown thanks to stinkbugs. I try not to spray but the beneficial insects have not done anything to stinkbugs. I am not sure spray helps either as they are not easy to kill. I don't spray my ornamentals at all and put up with holes here and there. The vegetable garden is where I can't grow anything with out bugs. Kale is full of holes but I don't like it anyway. Onions had spots on their leaves this spring; that may have been disease. I still got onions but not as nice as last year because it was dry this spring. Lettuce was the only pest free crop and the potato bugs didn't really hurt the potatoes underground. I sprayed them too along with the tomatoes when striped blister beetles came through like a plague. Maybe this is a sign for me to go back to flowers.

You might try a companion planting of radishes you can harvest some but let some bloom and go to seed this will keep some of the bad bugs at bay as they don't like them any plants that are having a caterpillar problem Bt will do the trick it is organic it affects the digestive system and they die in a day you just have to spray the area they are feeding on

I am afraid of Bt because I think Thuricide made me sick for two summers. Finally I figured out it was the spray not the tomatoes that were making me sick.

To bad about the Bt allergy it is the only good thing to use against caterpillars ,I try not to spray it on the fruit of the plant and always wash them off before consuming, I've heard of catching some of the pests smashing them up and adding water and spray that on the leaves to repel them. I have also propagated by own Bt you just find a caterpillar that has died from Bt and add him to some milk and let set for 2 days seal it in a jar and use it just like the stuff you bought.